iio:buffer.h Reformat structure comments to be inline.

This should make it easier to see how the structure is split into
public and private parts - reflected in the generated documentation.

Deliberately use /* instead of /** for the private elements to avoid
warnings when kernel-doc script runs.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>